pytest: fix flake in test_onionmessage_ratelimit.

By submitting them all at once, rather than serially, we should *definitely* hit the
ratelimit.  We can also reduce the timeout (from 60 seconds) to speed the test up.

```
    @pytest.mark.slow_test
    def test_onionmessage_ratelimit(node_factory):
        l1, l2 = node_factory.line_graph(2, fundchannel=False,
                                         opts={'allow_warning': True})

        offer = l2.rpc.call('offer', {'amount': '2msat',
                                      'description': 'simple test'})

        # Hopefully we can do this fast enough to reach ratelimit!
>       with pytest.raises(RpcError, match="Timeout waiting for response"):
E       Failed: DID NOT RAISE <class 'pyln.client.lightning.RpcError'>

tests/test_pay.py:5825: Failed
```

Signed-off-by: Rusty Russell <[email protected]>

Author: rustyrussell

tests/test_pay.py

@@ -5814,17 +5814,21 @@ def test_pay_legacy_forward(node_factory, bitcoind, executor):
 
 # CI is so slow under valgrind that this does not reach the ratelimit!
 @pytest.mark.slow_test
-def test_onionmessage_ratelimit(node_factory):
+def test_onionmessage_ratelimit(node_factory, executor):
     l1, l2 = node_factory.line_graph(2, fundchannel=False,
                                      opts={'allow_warning': True})
 
     offer = l2.rpc.call('offer', {'amount': '2msat',
                                   'description': 'simple test'})
 
     # Hopefully we can do this fast enough to reach ratelimit!
+    futs = []
+    for _ in range(8):
+        futs.append(executor.submit(l1.rpc.fetchinvoice, offer=offer['bolt12'], timeout=10))
+
     with pytest.raises(RpcError, match="Timeout waiting for response"):
-        for _ in range(8):
-            l1.rpc.fetchinvoice(offer['bolt12'])
+        for f in futs:
+            f.result(TIMEOUT)
 
     # Normally l2 gets upset, but actually l1 can get upset with replies!
     assert (l1.daemon.is_in_log('WARNING: Ratelimited onion_message: exceeded one per 250msec')